In Chlamydomonas reinhardtii cells, the amount of mRNA coding for the chlorophyll a/b binding proteins of photosystem II (cab II) oscillates in light/dark cycles and in constant dim light. This rhythmic behavior applies to the overall expression of the entire cab II gene family as well as to a single member of the family. The highest mRNA abundance is found in the middle of the subjective day and the lowest in the middle of the subjective night. In constant darkness the cab II mRNA rhythm damps rapidly. The cab II mRNA rhythm persists in non-growing cells under CO2-starvation conditions indicating that the cab II mRNA rhythm is not merely a consequence of cell division, although cell division may influence the amplitude of the cab II mRNA rhythm. The properties of this mRNA oscillation conform to all the major characteristics of circadian rhythms: the period in constant conditions is about 24 h, the rhythm entrains to 24 h light/dark cycles, and the period is temperature compensated. This report is the first demonstration of a circadian rhythm of cab II gene expression in single cells. beta-Tubulin mRNA also shows an oscillation in its abundance in LD cycles and in constant dim light, although its peak-to-trough amplitude is smaller than that of the cab II mRNA rhythm. The beta-tubulin mRNA rhythm peaks in the early night in LD cycles, but in constant illumination, it peaks at about the same circadian phase (i.e., mid-subjective day) as does the cab II mRNA rhythm. Finally, the amount of mRNA coding for mitochondrial cytochrome c is rhythmic in a light/dark cycle but is constant in constant dim light or constant darkness. Surprisingly, this mRNA exhibits a daily oscillation in constant dim light under the specific condition of CO2-depletion.